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Characteristics of thunderstorm-generated low altitude wind shear: a survey based on nationwide Terminal Doppler Weather Radar testbed measurements

Summary

The characteristics of microbursts and gust fronts, two forms of aviation-hazardous low altitude wind shear, are presented. Data were collected with a prototype terminal Doppler weather radar and a network of surface weather stations in Memphis, Huntsville, Denver, Kansas City, and Orlando. Regional differences and features that could be exploited in detection systems such as the associated reflectivity, surface wind shear, and temperature change are emphasized.
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Summary

The characteristics of microbursts and gust fronts, two forms of aviation-hazardous low altitude wind shear, are presented. Data were collected with a prototype terminal Doppler weather radar and a network of surface weather stations in Memphis, Huntsville, Denver, Kansas City, and Orlando. Regional differences and features that could be exploited...

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Microburst observability and frequency during 1988 in Denver, CO

Published in:
MIT Lincoln Laboratory Report ATC-170

Summary

The observability of microbursts with single-Doppler radar is investigated through comparison of radar data and surface weather sensor data. The data were collected during 1988 in Denver, CO as part of the FAA Terminal Doppler Weather Radar measurement program. Radar data were collected by both and S-band and C-band radar, while surface data were taken from a mesoscale network of 42 weather sensors in the vicinity of Denver's Stapleton International Airport. Results are compared with previous similar studies of observability using data from 1987 in Denver, and 1986 in Huntsville, AL. A total of 184 microbursts impacting the surface mesonet were identified. For those microbursts for which both radar and surface data were available, 97% were observable by single-Doppler radar. This compares to 94% observability during 1987 in Denver, and 98% during 1986 in Huntsville. Two strong microbursts (at lease 20 m/s differential velocity) were unobservable by radar throughout their lifetime: one due to low signal-to-noise ratio, and the other due initially to an asymmetric outflow with low signal-to-noise ratio also a contributing factor. Two other microbursts, with differential velocities from 10-19 m/s, were unobservable by radar: one due to shallow outflow with a depth limited to a height below that of the radar beam, and one due to asymmetric outflow oriented unfavorably with respect to the radar viewing angle. Consistent with previous observations, microburst occurrence was most frequent during June and July, when 94 microbursts were identified on 20 days. An anomalously high frequency was also seen in April, although the strength of these events was relatively modest. As expected, the diurnal distribution shows the late afternoon to be the most favorable time for microburst development; more than half of all events reached their maximum strength between the hours of 2-5 p.m. local time.
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Summary

The observability of microbursts with single-Doppler radar is investigated through comparison of radar data and surface weather sensor data. The data were collected during 1988 in Denver, CO as part of the FAA Terminal Doppler Weather Radar measurement program. Radar data were collected by both and S-band and C-band radar...

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Analysis of microburst observability with Doppler radar through comparison of radar and surface wind sensor data

Published in:
24th Conf. on Radar Meteorology, 27-31 March 1989, pp. 171-174.

Summary

As part of the FAA Terminal Weather Doppler Weather Radar (TDWR) measurement program in Huntsville, AL and Denver, CO during 1986 and 1987, respectively, the ability of a single Doppler weather radar to observe microburst outflow signatures (i.e., show identifiable radial velocity patterns) was assessed by comparing radar-observed microbursts with those identified by joint use of both radar and data from a mesoscale network (mesonet) of surface meteorological stations (Clark, 1988; DiStefano, 1988). Observability by radar must be considered together with pattern recognition algorithm performance for observable microbursts (Campbell et al., 1988) in order to fully assess the potential effectiveness of an automated microburst detection system which relies on data from a single Doppler radar. The comparison of radar and surface sensor data presented here investigates the possibility that some outflows may not be observable by radar due to: (1) low SNR (signal-to-noise ratio), (2) very shallow outflows for which the radar beam is scanning too high above the surface, (3) blockage of the beam, and/or (4) asymmetry in the surface outflow causing the radar to significantly underestimate the magnitude of the surface wind shear (Eilts and Doviak, 1987; GAO, 1987). Also addressed is the possibility that microbursts are not observed by the mesonet surface sensors because the spacing between stations is too great, or because the microburst outflow does not reach the surface due to a dense layer of cold air at the surface.
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Summary

As part of the FAA Terminal Weather Doppler Weather Radar (TDWR) measurement program in Huntsville, AL and Denver, CO during 1986 and 1987, respectively, the ability of a single Doppler weather radar to observe microburst outflow signatures (i.e., show identifiable radial velocity patterns) was assessed by comparing radar-observed microbursts with...

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Microburst recognition performance of TDWR operational testbed

Published in:
Proc. Third Int. Conf. on the Aviation Weather System, 30 January - 3 February 1989, pp. 25-30.

Summary

This paper describes current work in assessing the microburst recognition performance of the Terminal Doppler Weather Radar (TDWR) operational testbed. The paper is divided into three main sections: microburst recognition algorithm, performance assessment methodology and results. The first section provides an overview of the prototype TDWR microburst recognition algorithm The algorithm uses radar data from both surface scans and scans aloft to identify microburst events. The surface scan is used to identify microburst outflows, and the scans aloft provide information concerning reflectivity and velocity structures associated with microbursts to improve recognition rate and timeliness. The second section of the paper describes the methodology for assessing the recognition performance of the system. The performance of the testbed system is addressed from two viewpoints: radar detectability and pattern recognition capability. The issue of radar detectability is examined by comparing radar and mesonet data to determine if any events observed by the mesonet fail to be observed by the radar. The issue of pattern recognition performance is assessed by comparing microburst recognition algorithm outputs with truth as determined by expert radar meteorologists. The final section of the paper provides performance results for data collected by the testbed radar at Huntsville, AL and Denver, CO.
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Summary

This paper describes current work in assessing the microburst recognition performance of the Terminal Doppler Weather Radar (TDWR) operational testbed. The paper is divided into three main sections: microburst recognition algorithm, performance assessment methodology and results. The first section provides an overview of the prototype TDWR microburst recognition algorithm The...

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Observability of microbursts using Doppler weather radar and surface anemometers during 1987 in Denver, CO

Published in:
MIT Lincoln Laboratory Report ATC-161

Summary

This report focuses on the observability of microbursts using pulse Doppler weather radars and surface anemometers respectively by an experienced meterologist. The data used for this study were collected in the Denver, Colorado area during the FAA Terminal Doppler Weather Radar (TDWR) measurement program in 1987. The methods used for declaring a microburst from both Doppler radar and surface anemometer data are described. The main objective of this report is to compare the 1987 radar observed microbursts (which impacted the area covered by a surface anemometer system) with the surface mesonet observed microbursts. Of the 66 microbursts for which radar and mesonet data were available, 4 were not observed by the radar and 1 was not observed by the mesonet. All four microbursts not observed by the radar were classified as "dry" events with low surface reflectivities and with three of the four being relatively weak (peak velocity differences < 20 m/s) shear events. Possible reasons as to why these microbursts were not observed are discussed in detail. The strongest event exceeded 20 m/s (differential velocity) for two minutes and appears to have been missed due to a combination of very low reflectivity and a very shallow depth overflow.
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Summary

This report focuses on the observability of microbursts using pulse Doppler weather radars and surface anemometers respectively by an experienced meterologist. The data used for this study were collected in the Denver, Colorado area during the FAA Terminal Doppler Weather Radar (TDWR) measurement program in 1987. The methods used for...

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Study of microburst detection performance during 1985 in Memphis, TN

Published in:
MIT Lincoln Laboratory Report ATC-142

Summary

This report focuses on the detectability of microbursts using pulse Doppler weather radars and surface anemometers. The data used for this study were collected in the Memphis, TN area during the FLOWS project of 1985. The methods used for declaring a microburst from both Doppler radar and surface anemometer data are described. The main objective of this report was to identify the results that were generated by comparing the 1985 radar detected microbursts (which impacted the surface anemometer system) wit the surface mesonet detected microbursts. In so doing, the issue of missed microburst detections, for which there occurred two (both by the radar), is identified. Possible reasons as to why there two microbursts were not detected are discussed in detail.
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Summary

This report focuses on the detectability of microbursts using pulse Doppler weather radars and surface anemometers. The data used for this study were collected in the Memphis, TN area during the FLOWS project of 1985. The methods used for declaring a microburst from both Doppler radar and surface anemometer data...

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Preliminary Memphis FAA/Lincoln Laboratory operational weather studies results

Published in:
MIT Lincoln Laboratory Report ATC-141

Summary

During 1984 and 1985 M.I.T. Lincoln Laboratory, under the sponsorship of the Federal Aviation Administration (FAA) conducted a measurement program in the Memphis, Tennessee, area to study low-level wind shear events and other weather phenomena that are potentially hazardous to aircraft operations, with particular emphasis on those issues related to the Terminal Doppler Weather Radar (TDWR). The principal sensor for the measurement program was the S-band FAA-Lincoln Laboratory Testbed Doppler Weather Radar (FL2) which incorporates many of the functional features of the TDWR. Both FL2 and a C-band Doppler Weather Radar operated by the University of North Dakota (UND) obtained reflectivity, mean velocity and spectrum width measurements with a radar geometry and scan sequences to facilitate determining the surface outflow features of microbursts at the anticipated TDWR ranges. A 30-station network of automatic weather stations (mesonet) collected I-min averages of temperature, humidity, pressure, wind speed and direction, and total rainfall, plus the peak wind speed during each minute; this system operated from about March through November 1984 and 1985. Finally, the UND Citation aircraft operated two 3-week periods during 1985, collecting thermodynamical, kinematical and microphysical data within and around selected storms in the area as well as providing in situ truth for locations and intensity of turbulence. This report describes the principal initial results from the Memphis operations, stressing the results from 1985 when the FL2 radar was fully operational. These results are compared to those from previous studies of wind-shear programs, e.g., NIMROD near Chicago, JAWS and CLAWS near Denver. During 1985, 102 microbursts were identified in real time along with 81 gust fronts. One of the dominant results is that most microbursts in the mid-south are wet; that is, they are accompanied by significant rainfall. This is in contrast, for example, to the results from Denver where more than half of all microbursts have little or no appreciable rain reaching the ground. Aside from this major difference, microbursts near Memphis were similar to those found elsewhere in the country in terms of wind shear magnitude. The report also gives more representative results from the aircraft operations and discusses the effectiveness of the ground-clutter filters used on the FL2 radar.
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Summary

During 1984 and 1985 M.I.T. Lincoln Laboratory, under the sponsorship of the Federal Aviation Administration (FAA) conducted a measurement program in the Memphis, Tennessee, area to study low-level wind shear events and other weather phenomena that are potentially hazardous to aircraft operations, with particular emphasis on those issues related to...

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The Flows (FAA-Lincoln Laboratory Operational Weather Studies) automatic weather station network in operation

Published in:
MIT Lincoln Laboratory Report ATC-134

Summary

This report describes in detail the FLOWS automatic wearhter station network as a wind shear data collection system, describes the post-data collection processing and analysis procedures, and presents an evaluation of the performance of the network after its first field test and an illustration of how the data are actually used in analysis. Preliminary results of an investigation into correcting wind speed measurements for the effects of anemometer site obstructions are also presented. The goal has been to describe all facets of the weather station system, data processing, and preliminary performance results in sufficient detail so as to allow current and future users to understand the experimental utility of the weather station data and the limitations, the characteristics of the meterological sensors, and the possible improvements that could be made to the weather station system.
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Summary

This report describes in detail the FLOWS automatic wearhter station network as a wind shear data collection system, describes the post-data collection processing and analysis procedures, and presents an evaluation of the performance of the network after its first field test and an illustration of how the data are actually...

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Low-altitude wind shear characteristics in the Memphis, TN area based on mesonet and LLWAS data

Published in:
Proc. 14th Conf. on Severe Local Storms, 29 October -1 November 1985, pp. 322-327.

Summary

As part of the 1984-85 FLOWS (FAA-Lincoln Laboratory Operational Weather Studies) Project, mesonet and Doppler radar data are being collected on rain and thunderstorms in the Memphis, TN area. One of the key goals of the FLOWS Project is to characterize and evaluate the various form of potentially aviation-hazardous low-altitude wind shear in parts of the country where this type of high spatial and temporal resolution meteorological data have not previously been collected. The 1982 JAWS (Joint Airport Weather Studies) Project revealed that the "microburst", a small scale, intense downdraft which hits the surface and causes a strong divergent outflow of wind, has been the source of much of the hazardous wind shear in the Denver area. The 1978 NIMROD (Northern Illinois Meteorological Research on Downbursts) Project revealed that microbursts occur there on convectively unstable days along with gust fronts and "macrobursts" (scale 4-40 km). Other experiments have largely failed to detect microbursts because their observational networks have not been dense enough to resolve this small scale. A compilation of pioneering studies of microburst-related aircraft accidents around the world by Fujita (1985) illustrates clearly the inherent danger of the microburst wind pattern to jet aircraft, wherever it occurs. In developing ways to best meet the goal of providing warning and protection from low-altitude wind shear in the airport terminal areas, the FAA will need to characterize the problem in different parts of the country. It may be misleading, for example, to use the results on wind shear in the Denver area, or any other single geographical locale, to typify the requirements for microburst warnings at all airports in the country. An important region in terms of its frequency of commercial air traffic control and of thunderstorms, in which high resolution measurements capable of revealing microburts have never before been collected, is the southeastern part of the United States (excluding Florida). During 1984 Lincoln Laboratory continuously collected surface meteorological data from 25-30 mesonet stations and FAA Low Level Wind Shear Alert System (LLWAS) data from the 6 anemometers at the Memphis International Airport from May through November (212 days total). Presented here are preliminary results on the characteristics of wind shear events in the Memphis area. Microburst statistics for Memphis are contrasted with those computed by Fujita and Wakimoto (1983) for the Denver area during JAWS and the Chicago area during NUMROD. A detailed analysis of a microburst that occurred on August 11, 1984 is also presented.
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Summary

As part of the 1984-85 FLOWS (FAA-Lincoln Laboratory Operational Weather Studies) Project, mesonet and Doppler radar data are being collected on rain and thunderstorms in the Memphis, TN area. One of the key goals of the FLOWS Project is to characterize and evaluate the various form of potentially aviation-hazardous low-altitude...

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An automatic weather station network for low-altitude wind shear investigations

Published in:
MIT Lincoln Laboratory Report ATC-128

Summary

During the summer of 1983 an experimental network of automatic weather stations (a mesonet) was operated in the vicinity of Hanscom Field, northwest of Boston, as part of a larger effort to collect Doppler radar and meteorological data on thunderstorms and other potentially hazardous weather events in this area. This report describes the mesonet system used and presents in detail the data collected on 21-22 July 1983. Conclusions about the limitations and the future use of the mesonet system are also included.
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Summary

During the summer of 1983 an experimental network of automatic weather stations (a mesonet) was operated in the vicinity of Hanscom Field, northwest of Boston, as part of a larger effort to collect Doppler radar and meteorological data on thunderstorms and other potentially hazardous weather events in this area. This...

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